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Investments - Amortized Cost and Fair Value of the Auction Rate Securities (Details) - Auction rate securities - USD ($)
$ in Thousands
Apr. 30, 2017
Apr. 30, 2016
Amortized cost of available-for-sale securities by contractual maturity    
Due one through five years $ 700  
Due after 10 years 2,000  
Total 2,700 $ 3,100
Fair value of available-for-sale securities by contractual maturity    
Due one through five years 701  
Due after 10 years 1,796  
Total $ 2,497 $ 2,780